Elizabeth Williamson and Dante Chinni have an interesting analysis in the Wall Street Journal this week of the older, inner-ring suburbs around Washington, Denver, Atlanta and other big cities that were once reliable centers of conservative politics. In the last decade, these places have been changing. Highly educated newcomers are moving in. Median incomes are rising. Upscale restaurants and mixed-use developments of the kind you might find in Reston Town Center are popping up.

In the process, these places are growing denser:

These neighborhoods—so-called mature suburbs that sprouted in the decades after World War II—have become so densely populated over the past decade that they more closely resemble the big cities nearby. The U.S. census now classifies the counties that contain them as "urban."
The population of mature suburbs in the U.S. grew to about 60 million in 2010 from about 51 million a decade earlier, according to a Wall Street Journal analysis of census data.
The newer residents look, shop and vote more like urban dwellers than suburbanites of the past.

And, in fact, results from the last two presidential elections suggest that these suburbs are voting more like cities -- in short, they're leaning more Democratic. This good graphic from the story illustrates the dominance of Democrats in big, dense cities, the comparable dominance of Republicans in the sparsely populated exurbs, and a narrowing gap in the communities in between. In that middle ground closer to the urban core, many "mature suburb" counties in the Journal's analysis have actually flipped from red to blue in the last two national election cycles.

This pattern reflects a fascinating larger relationship between population density and politics that was on particular display in the 2012 election: Dems overwhelmingly have a lock on the kinds of places where people live close together, Republicans on the kinds of places where people are more spread out. In 2012 Barack Obama won 49 of the 50 densest counties in the U.S., and Mitt Romney 49 of the 50 least dense. That trend holds steady even beyond the extremes of the density spectrum.

This also suggests that there might be a tipping point somewhere in between the poles of crowded Manhattan and rural Montana, a density below which people are more likely to vote Republican, and above which they're more likely to vote Democratic.

That would be the point around which a previously red county -- say, an older suburb -- might become dense enough as to lean blue in an election. Looking back at 2012 election results by county, software developer Dave Troy has suggested that this might occur around a density of about 800 people per square mile:

So where do the tipping point counties that Williamson and Chinni identified fall? Their story focuses on some rapidly growing suburbs around Denver, Washington, Atlanta and Columbus, Ohio. One was Loudoun County, Va., which voted for Bush in 2004. In 2012, Loudoun gave Obama 51.6 percent of its vote. Its population density based on 2012 estimates: 654 people per square mile. Its density barely a decade earlier in 2000: 329 people per square mile.

Arapahoe County (747 people per square mile in 2012 and growing) and Jefferson County (713, ditto) in Colorado both voted for Obama in 2012 but Bush in 2004. Same with Douglas County (670) and Rockdale County (660) in Georgia, outside of Atlanta. From 2000 to 2010, Douglas County's population grew by more than 40 percent, Rockdale's by more than 20 percent.

Those numbers would suggest that these counties are near or approaching the political tipping point density on the graph above. And they reinforce the idea that places aren't permanently affixed on that continuum. Population density will change over time, as Williamson and Chinni observe, and so will politics.

But why exactly are population density and politics linked in this way? Are Democrats more likely to self-select for denser places and Republicans for less-dense ones? Or does density itself create policy concerns -- like the need for mass transit, affordable housing, and anti-poverty programs -- that are more naturally addressed by Democrats?